The present study investigates the longitudinal relationships between bullying roles (bullying, passive bystanding, victimization) and moral disengagement to examine influences on the shifting role of bullying in Japanese middle school students. Participants were 271 Japanese students (Time 1: M age = 12.72, SD = 0.45, 136 boys and 135 girls) recruited from two public middle schools (9 classes). Five data collection occasions in Grades 7 to 9 alternated between surveys of self-reported bullying behavior (three timepoints) and moral disengagement (two timepoints). Structural equation modeling showed that moral disengagement was reciprocally related to bullying and passive bystanding. Victimization was only related to moral disengagement in Grade 9, such that moral disengagement in June predicted increased victimization experiences in December. These findings suggest that intervention and prevention programs in schools need to actively address moral disengagement, with a focus on bystanders as well as perpetrators and victims of bullying.